While a wide variety of fibrous materials have been tried as air filter media, particularly for removing particulate matter and condensed tars from cigarette smoke, the most frequently used have been cellulose derivatives such as crimped textile tow of cellulose acetate fiber or mixture of the types described, for instance, in U.S. Pat. Nos. 2,794,239 and 4,366,826.
While such material is generally not as efficient as some of the finer denier synthetic fibers or fiber mixtures such as polyolefins, there remain substantial cost and handling advantages. For example, cellulose acetate tow can be rapidly and continuously processed into cuttable (stable) filter rods using standard filter-making equipment without jamming of the open fiber tow. This is accomplished by applying non-volatile liquid organic plasticizers such as triacetin, diacetin or citric acid in place of conventional adhesives to restrict lateral and longitudinal movement of the fibers. For such purpose the plasticizer is conveniently sprayed onto an open moving tow of cellulose acetate as fine droplets which are absorbed into the fibers relatively slowly and form softened areas capable of bonding adjacent fibers. This random bonding of fibers throughout the tow produces a consistency which is found useful for cutting filter rods into suitable lengths or plugs.
The above advantages, however, are offset by certain limitations. For example, cellulose acetate fibers are inherently weak (1.0-1.2 g./denier) compared with many synthetic fibers. This, in turn, limits the amount of tension and crimp which can be applied to cellulose acetate fiber tow on a conventional filter rod make-up machine. By their nature, cellulose acetate filaments cannot be drawn to obtain a denier of less than about 1.5.
Highly drawable synthetic polyolefin fiber, such as polypropylene, on the other hand, can be easily drawn from 3.5 denier or higher to a denier of less than 1 denier with correspondingly increased filter efficiency.
Crimped polyolefin fibers such as polypropylene, in particular, possess strength and durability while retaining an acceptable level of pressure drop. Using such filaments, it is possible to meet the need for finer fiber tows having a suitable crimp and a high tensile strength.
Polyolefin fibers such as polypropylene, however, also have some serious disadvantages. These stem mainly from the fact that the above-described method of bonding an open or bloomed fiber tow of cellulose acetate by using non-volatile liquid organic plasticizers, will not work with polyolefins due to the relative insolubility of the fibers. Moreover, conventional adhesives fail to supply the necessary consistency or hardener effect as indicated above. To meet this problem, an additional heat fusion step has met with some success. Such added step, however, requires substantial changes in conventional filter-making operations and equipment, and is expensive. For example, a heavier plug wrap, tipping paper and more effective gluing system are required for sealing the wrapper around the fiber.
It is an object of the present invention to effectively improve the dimensional stability and resiliency of polyolefin fiber filter elements.
It is a further object to obtain a stabilizing formulation capable of wetting polyolefin fibers such as polypropylene without jamming the open towed fiber within conventional filter processing and cutting equipment.
A still further object is to obtain a polypropylene fiber tow air filter suitable, in crush resistant and drawing properties, for use as a cigarette filter.